Traveling wave tube interaction circuit manufacture

ABSTRACT

A helix slow wave propagation circuit supported by longitudinally aligned ceramic rods is brazed to the rods by providing seal-forming surfaces on the rods and a plating of brazing material on the helix.

United States Patent n 1 Bottcher et al.

1451 July 31, 1973 I TRAVELING WAVE TUBE INTERACTION CIRCUIT MANUFACTURE Inventors: James II. Bottcher; Rohert l Pyles, both of Gainesville; John L. Rawls, Jr., Archer, all of Fla.

Assignee: Sperry Rand Corporation,

New York, NY. Filed: Mar. 7, 1972 Appl. No.: 232,563

us. 01 29/600, 29/4717, 29 4727, 29/4731, sis/3.5 im. on. H0lp 11/00 Field of Search 29/600, 601, 473.1, 29/47l.7, 472.7; 31513.5

[56] References Cited UNITED STATES PATENTS I 3,475,643 10/ I969 Schrager 315/35 3,548,345- l2/l970 Falce'; ..29/600X Primary Examiner-Charles W. Lanham Assistant Examiner--Robert M. Rogers Attorney-Howard P. Terry [57] ABSTRACT A helix slow wave propagation circuit supported by longitudinally aligned ceramic rods is brazed to the rods by providing seal-forming surfaces on the rods and a plating of brazing material on the helix.

7 Claims, JDrawing Figures TRAVELING WAVE TUBE INTERACTION CIRCUIT MANUFACTURE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention pertains to a method of manufacture of improved high frequency, high power traveling wave tubes and more particularly relates to manufacture of an integrated helix slow wave propagation structure and ceramic support rod assembly particularly adapted for affording ready flow of heat from the slow wave structure to exterior parts of the vacuum tube.

2. Description of the Prior Art Generally, prior art velocity modulation tubes, especially those of the helix traveling wave type, have been severely limited as to maximum operating power level because of heating of the slow wave interaction helix circuit by those few electrons which are actually intercepted by the helix. While the efficiency of magnetic focusing systems has been greatly improved, higher voltages and beam currents are employed, and a severe limitation has still been imposed by the point contact relation between each turn of the helix and each ceramic support rod. Such support arrangements are characterized by a very high impedance to heat fiow from the helix to exteior parts of the vacuum tube.

- They are therefore found unsatisfactory for use'in high power devices.

SUMMARY OF THE INVENTION The present invention is an improvement in the method of manufacture of helix traveling wave power amplifier and oscillator tubes. The invention includes at least steps of cleaning ceramic support rods and generating on them seal-forming surfaces, of cleaning and plating a helix interaction circuit, of binding the rods and helix in an assembly, of heating the assembly to form brazed connections between the helix and the ceramic rods, and of removal of excess seal forming and plating materials to provide the finished assembly. The product of the method is a helix supported by longitudinally aligned ceramic rods with brazed connections at the rod contacting points with individual turns of the helix. A greatly improved rate of heat flow is thus provided from the helix into the support rods and thus to the exterior of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section view of a helix traveling wave amplifier incorporating a helix-helix support arrangement made according to the novel method of the present invention.

FIG. 2 is a perspective view of a jig for practising the novel invention.

FIG. 3 is a fragmentary cross section view of a portion of the structure of FIG. 1 on an enlarged scale more readily permitting illustration of thin material layers used in practising the novel method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents a cross section view of an electron beam high frequency interaction circuit 1 and vacuum envelope 2 generally similar to those elements as employed in the past in conventional helix traveling wave amplifier or oscillator tubes. Helical interaction circuits like helix 1 on which slow waves readily propagate have customarily been directly supported within a vacuum envelope 2 by three or more longitudinally extending electrical insulator rods, such as rods 3, 4, and 5. Rods 3, 4, 5 usually extend parallel to the tube axis6, which is also the axis of helix 1, and are often composed of electrical insulating ceramic such as aluminum oxide. According to the present invention, the rods 3, 4, and 5 and helix'l are incorporated into a unitary structure by an improved method generating fillets of brazing material, such as at 7, 8, and 9, only between the respective rods 3, 4, 5 and adjacent part of the wire or tape of the helix 1. With rods 3, 4, 5 consisting of a ma terial such as beryllium oxide that is both a good electrical insulator and a good heat conductor, the fillets 7, 8, 9 at each contact point with helix 1 enable the assembly to have a much higher capability of conducting heat from the refractory metal helix 1 to vacuum envelope 2 for ultimate dissipation exterior of the vacuum tube. Since the maximum power operating level of the traveling wave tube is ultimately limited by helix temperature rise caused by intercepted electrons straying from the electron beam, efficient condition of heat from helix 1 to the exterior of the vacuum tube is highly desirable.

Preparatory to making use of them, each beryllium oxide rod 3, 4, or 5 to be used in a new tube is cleaned to remove oil, grease, and other surface contaminants by a series of cleaning steps which may be conventional. One cleaning method found suitable involves use of a conventional vapor degreasing step; immersion of the rod inthe degreasing fluid is avoided. This initial step may be followed by brief immersion in a mild alkali solution, in nitric acid, and in concentrated hydrochloric acid, interspersed with clean water rinses. After soaking the rod in hot deionized water at C. for 30 minutes, the rod is next dipped briefly in methyl alcohol. The final immediate treatment is baking in air at 900 C. for 30 minutes in a ceramic boat in a conventional electric furnace. Other cleaning processes may be successfully used which vary in detail from the foregoing.

Each beryllium oxide rod 3, 4, 5 is next provided with a bonded longitudinal strip of sintered seal-forming material in the region which is ultimately to contact helix 1. A film-forming slurry is applied by brushing in a generally conventional manner, such as in the manner described by R. W. Buck in the U.S. Pat. No. 3,590,468 for a Glassy Phase Method for Making Pure Aluminato-Metal Hermetic Seals, issued July 6, 1971 and assigned to the Sperry Rand Corporation. Alternatively, a refined method operating at lower temperatures and thus considered particularly compatible with the nature of beryllium oxide is employed, such as that described by MW. White in the U.S. Pat. application Ser. No. 86l,454 for a Method for Metalizing Beryllium Oxide at Low Temperatures," filed Sept. 26, 1969, and also assigned to the Sperry Rand Corporation. Alternatively, the coat of slurry may be applied by dipping the entire rod in the film forming slurry.

For example, the seal-forming film strip may be made from a slurry comprising a butyl alcohol suspension of molybdenum trioxide and manganese dioxide, a glass frit, and a nitrocellulose binder with a small amount of cuprous oxide. The slurry strip is bonded to the beryl:

.lium oxide as a seal-forming strip by sintering at a temperature of I,l00 C., for example, for substantially 20 minutes. The firing atmosphere is a wet dissociated ammonia or other highly reducing atmosphere of conventional type. After cooling, any loose material may be removed by burnishing with a stainless steel wire brush so that the seal-forming strip film has a smooth and uniform surface. The application of the slurry, sintering, and burnishing will generally be repeated one or more times.

After the final sintering and burnishing, a metal coating is bonded to the sintered film in a conventional manner. For example, a slurry of nickel oxide in an alcohol suspension may be applied to the sintered strip and then reduced to a nickel film firmly adhering to the seal-forming strip by heating in a wet hydrogen or other reducing atmosphere at a temperature of l,l C. for a period of 10 minutes. Such an adherent nickel film presents a surface which may readily be brazed to helix 1.

To prepare the molybdenum helix 1, it is plated to a depth of 0.0003 to 0.0005 inches with metal after having been formed from wire or tape and cleaned by conventional processes. A plate material suitable for acting as a brazing material isused, and also having high electrical und thermal conductivities, such as copper, silver, or preferably gold.

Preparatory to brazing, the beryllium oxide rods 3, 4, 5 and helix 1 are mounted in the'brazing jig of FIG. 2. The jig consists of a base with similar opposed end cage holders 11 and 12, one or both of which may be supplied with a means for permitting the cage holders ll, 12, which are constructed of oxidized stainless steel, to be translated along base 10. For example, cage holder 12 may be translated to a new position by first loosening, then tightening, the nut 14 on a bolt 15 extending in slot 13 through base 10; other conventionAl mechanisms for the purpose may be readily envisioned by those skilled in the art.

- Each cage holder 11 and 12 is provided with holes, as at 16 and 17, for accepting the ends of the beryllium oxide rods. Also, each cage holder is provided with a central hole 18 for accommodating helix mandrel 20 around which the gold plated helix 1 has been placed after its formation. The mandrel 20 and helix 1 are inserted in cage holder 11; then, the rods 3, 4, and 5 are introduced in such a manner that the nickel plated strips of each rod are in contact with the turns of the helix.

The brazing jig is also supplied with a slide block 22 which may be translated longitudinally along the surface 23 of base 10 and is arranged to surround the assembly 24 of rods 3, 4, 5, helix 1, and mandrel 20 in a close fitting manner. The'slide block 22 is progressively moved along the assembly 24 from cage holder 12, for example, to cage holder 11, thus maintaining the concentricity of the assembly 24. As the slide block is progressively moved, the assembly 24 is wrapped with securing molybdenum wires, such as wire 25, which wires are then individually twisted, as at 26. The twisted wires will be provided, for example, at one inch intervals along the assembly 24. Having, by means of wires such as 25, 26, bound the assembly parts in fixed interrelation, assembly 24 is removed from the jig by first moving cage holder 12 away from the assembly 24, permitting its withdrawal from holes 16, 17, 18, for instance. Mandrel 20 is then removed from within helix 1.

The wired assembly 24 is now ready for brazing and is placed in horizontal position in a conventional brazing furnace. The brazing time and temperature, if gold plating is used will, for instance, be about 10 minutes at l,l00 C., preferably in an atmosphere of wet cracked ammonia such as is often used where a strongly reducing atmosphere is desired. In this step, the gold or other plating on helix 1 is melted. It does not diffuse to any remarkable degree into the molybdenum of helix 1, but alloys readily at the surface of the nickel strips on rods 3, 4, 5. In view of the surface and interfacial tension characteristics of the molten gold, it flows toward each of ceramic rods 3, 4, 5, forming the desired fillets 7, 8, and 9. These fillets, as previously noted, greatly increase heat conduction between helix 1 and the beryllium oxide rods 3, 4, 5. After assembly 24 is cooled, wires 25, 26 may be cut and removed.

Any gold remaining on helix 1 is not desired; neither are the portions of the nickel strips on parts of rods 3, 4, 5 not lying under the respective newly fonned fillets 7, 8, 9. Etching in a conventional manner removes all of the thin layer of gold which has not migrated to fillets 7, 8, 9 also incidentally but harmlessly removing a minor surface layer of gold from the fillets. The brazed assembly may be immersed for this purpose in a goldstripping chemical of conventional type. Various known organic cyanide complexes are found useful for removing gold plate. An aqueous solution of sodium cyanide and sodium hydroxide is also known to be suitable. A solution of sodium cyanide and hydrogen peroxide has been used successfully.

To remove all metal bonded between helix turns to rods 3, 4, 5, except that covered by fillets 7, 8, and 9, Q

the assembly 24 may be immersed for 15 seconds at 60 to C. in an acid material often used as a bright dip for removing oxides from ferro-chrome alloys used extensively in vacuum tubes. Immediate rinsing with water is required; after rinsing and inspection, the process may be repeated, if desired. A suitable etchant consists of 50 percent by volume sulfuric acid (66 Be.) and 25 percent by volume nitric acid (42 Be.), the remainder being water. After a final dip in dilute hydrochloric acid to remove any miscellaneous oxides, the assembly 24 is rinsed in deionized water, is forced air dried, and is then ready for installation in a vacuum tube.

The detailed structure formed by the novel process is shown in FIG. 3, which is an enlarged view of the portion of the assembly connecting one turn of helix 1 to the beryllium oxide rod 4. In this somewhat idealized representation, the surface of rod 4 is shown covered under gold fillet 8 adjacent helix 1 with an arcuate sealforming layer 30 which may penetrate slightly into the material of rod 1, also rising slightly above its surface. Covering the outer surface of seal-forming layer 30 is a thin layer of nickel 31. The gold fillet 8 forms a bond to nickel layer 31 and also to the illustrated turn of helix 1. In actual practice, the interface between sealforming layer 30 and rod 4 and the interface between layer 30 and nickel layer 31 may not be assharply defined as implied by the drawing, as will be recognized by those skilled in the art. It will also be understood by those skilled in the art that, before incorporation in a vacuum tube, the brazed assembly will be supplied with suitable attenuators in the usual manner for isolating the input and output sections of the helix.

It is seen that oxide helix novel method provides a very effective and inexpensive way of manufacture of a helix-helix support rod assembly particularly adapted for efficient heat transfer from the high frequency interaction structure to the exterior of the traveling wave tube. The method provides greatly increased thermal transfer between the helix and beryllium oxidehelix support rods and, being simple, readily lends itself to production use.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

We claim:

1. The method of manufacture of 'a traveling wave interaction circuit comprising the steps of:

generating on an elongated surface of a ceramic rod a longitudinally extending layer of metal bonded to said ceramic, plating a slow wave propagation structure with a layer adapted for use as a brazing material,

melting said brazing material layer for providing heat conducting fillets of said brazing material between portions of said slow wave propagation structure and portions of said generated longitudinally extending layer when cooled,

removing after cooling said plated metal layer not formed into said fillets, and

removing said generated longitudinally extending layer where not covered by said fillets.

2. The method as described in claim 1 wherein the step of generating said longitudinally extending layer of rial bonded to said ceramic, and generating on said layer of seal-forming material a longitudinally extending layer of nickel bonded to said seal-forming layer. g 3. The method as described in claim 1 wherein the step of plating a slow wave propagation structure with a layer adapted for use as a brazing material comprises plating said structure with gold.

4. The method as described in claim 1 additionally including the substep of:

temporarily binding in substantially parallel spaced relation a plurality of said ceramic rods about said slow wave propagation structure, each of said ceramic rods when bound having a respective generated longitudinally extending layer of metal in contact with said brazing material layer of said slow wave propagation structure. 5. The method as described in claim 4 additionally including the steps of:

heating said bound plurality of ceramic rods and said slow wave propagation structure in a reducing atmosphere for forming fillets of said brazing material at the contacts between said longitudinally extending layers of metal and said brazing material layer of said slow wave propagation structure.

6. The method as described in claim 1 wherein said step of removing said plated metal layer not formed into said fillets comprises immersing at least said slow wave propagation structure in an aqueous solution of a reagent for dissolving gold.

7. The method as described in claim 1 wherein the step of removing said generated longitudinally extending layer where not covered by said fillets comprises immersing at least said ceramic rod in an aqueous acid solution for removing nickel. 

2. The method As described in claim 1 wherein the step of generating said longitudinally extending layer of metal comprises the sub-steps of: generating on said elongated surface of said ceramic rod a longitudinally extending layer of seal-forming material bonded to said ceramic, and generating on said layer of seal-forming material a longitudinally extending layer of nickel bonded to said seal-forming layer.
 3. The method as described in claim 1 wherein the step of plating a slow wave propagation structure with a layer adapted for use as a brazing material comprises plating said structure with gold.
 4. The method as described in claim 1 additionally including the substep of: temporarily binding in substantially parallel spaced relation a plurality of said ceramic rods about said slow wave propagation structure, each of said ceramic rods when bound having a respective generated longitudinally extending layer of metal in contact with said brazing material layer of said slow wave propagation structure.
 5. The method as described in claim 4 additionally including the steps of: heating said bound plurality of ceramic rods and said slow wave propagation structure in a reducing atmosphere for forming fillets of said brazing material at the contacts between said longitudinally extending layers of metal and said brazing material layer of said slow wave propagation structure.
 6. The method as described in claim 1 wherein said step of removing said plated metal layer not formed into said fillets comprises immersing at least said slow wave propagation structure in an aqueous solution of a reagent for dissolving gold.
 7. The method as described in claim 1 wherein the step of removing said generated longitudinally extending layer where not covered by said fillets comprises immersing at least said ceramic rod in an aqueous acid solution for removing nickel. 